Radio frequency identification (RFID) systems are used for identification and/or tracking of equipment or inventory such as pallets, trucks, dollies or boxes or even the whereabouts of some animals, such as livestock in certain situations. These RFID systems are radio communication systems in which communications is provided between a radio transceiver, or interrogator, and a number of small, identifying labels or tags. These tags are read while in the radiation pattern or field of the interrogator, which may be connected to a computer-based tracking system. The intent of an RFID system is to provide a reliable and secure architecture that meets a predetermined performance requirement, while minimizing the cost of the interrogator and the tags. In the operation of RFID systems, the interrogator transmits to the tags using modulated radio signals, and the tags respond by transmitting modulated radio signals back to the interrogator. Specifically, the interrogator first transmits an amplitude modulated signal to the tag. Next, the interrogator transmits a continuous-wave (CW) radio signal to the tag. The tag then modulates the CW signal using modulated back scattering (MBS) wherein the antenna is electrically switched, by the tag's modulating signal, from being an absorber of radio frequency (RF) radiation to being a reflector of RF radiation; thereby encoding the tag's information onto the CW radio signal. The interrogator demodulates the incoming modulated radio signal and decodes the tag's information message.
The performance of an RFID system and, more specifically, a tag within such system is influenced by its surrounding environment An antenna in the tag, which is optimized for operation without nearby reflectors or dielectric absorbers, will not perform as effectively when those influences are near. Thus, system performance and sensitivity can be strongly affected by variations in the environment.
Many antenna configurations have been proposed for use in tags that operate in RFID systems. Some of these configurations are described in the following U.S. Pat. Nos. 4,853,705 (Landt); 4,816,839 (Landt); 4,782,345 (Landt); 4,724,443 (Nysen); and 5,394,159 (Schneider). Most of these configurations are dipole antennas, or tapered antennas which radiate from both sides of a tag containing such an antenna. These radiation patterns are therefore significantly altered when the tag is placed in close proximity to a reflecting or absorbing surface. For example, Nysen, in U.S. Pat. No. 4,724,443, proposes a patch antenna that has a quarter-wave strip line feed element. However, this configuration is complex, expensive to fabricate, and requires three levels of metallization. Although Schneider, in U.S. Pat. No. 5,294,159, describes a patch antenna design with only two layers of metalization and also describes a technique to match the antenna to a demodulating circuit, Schneider is not concerned with the problem of how the performance of a tag is affected by its surrounding environment. It is therefore desirable to provide in a tag an antenna that produces a near uniform performance irrespective of variations in those environments in which the tag operates.